1. Field of the Invention
The present invention relates to readout control of image signals obtained in an image sensor of a digital camera.
2. Description of the Background Art
On typical digital cameras, a solid-state image sensor such as a CCD (Charge Coupled Device) stores a subject image as signal charge and converts the charge into an electric image signal. Through signal processing such as amplification, the electric signal is displayed on a display such as CRT or stored as an image file. For such digital cameras, one of the factors determining the brightness of an output image is the exposure time. A short exposure time darkens the overall output image, whereas an excessively long exposure time causes accumulated signal charge in the solid-state image sensor to reach a level of saturation beyond a storage capacity, thereby making it difficult to identify the output image.
Japanese Patent Laid-open No. 8-31994 for example discloses a conventional technique for adjusting the exposure time. This technique is to control the time to accumulate signal charge automatically in accordance with varying illumination of a subject to thereby adjust an output image at a predetermined level.
Such automatic control of the conventional technique, however, prevents a user from capturing an image at desired exposure. This is the first problem of conventional digital cameras.
The second problem is caused from the intrinsic character of CCD cells. In the use of the CCD, pixel data are read out in sequence according to a pixel array of the CCD. In this case, a selective readout of only specific pixel data is impossible.
On the other hand, most of digital cameras perform preliminary imaging for automatic exposure (AE) and automatic focusing (AF) prior to actual imaging for recording or for display of a subject image on a liquid crystal display (LCD). Even in such preliminary imaging, pixel data are read out from all pixels because of the use of the CCD. On display, however, only pixel data left after data skipping are displayed since the LCD provided in a digital camera generally has a smaller number of pixels (e.g., about a half) than the CCD array in the image sensor. In addition, it is often unnecessary to use all pixel data in AE and AF operations.
On a conventional digital camera, all pixel data is read out from the image sensor even in the preliminary imaging that requires not all data. Accordingly, a readout of pixel data is time consuming, thereby delaying the actual imaging.